This invention relates to charging means which cause a toner for developing electrostatic latent image to charge.
A toner is charged positively or negatively according to the polarity of electrostatic latent images.
Only tribo-chargeability of a toner-constituting resin may be utilized to charge a toner. However, as sufficient charge amounts are not provided by this method, copied images become unclear and many fogs generate in the copied images. Therefore, a charge controlling agent such as a dye, a pigment or the like which reinforces chargeability is added in general to a toner to achieve a desired level of charge amounts.
However, particles of a charge controlling agent must be cropped out on a surface of a toner in order to enhance chargeability of the toner by adding these additives into the toner. Therefore, particles of additives drop out of toner surfaces by the friction between toner particles themselves, the collision between a toner and a carrier, the friction between a toner and a support of electrostatic latent images or the like, to pollute a carrier, a support of electrostatic latent images, a belt for a photosensitive member, a drum for a photosensitive member. As a result, chargeability becomes poor, and further the repetition of a copying process causes problems such as the decline in image density, the deterioration of reproducability of fine lines, the increase of fogs and the like.
It is proposed that the improvement of charging properties of a toner is achieved not only by the addition of additives into a toner but also by a transporting member, a toner-levelling member or a friction member (hereinafter, a material or a member which contacts with a toner in a developing process or prior to a developing process to provide a toner with charges necessary for development or to provide a toner with charge auxiliaryly is referred to as "charging means" in a general term, including the transporting member, the toner-levelling member and the friction member.)
Charging means can provide a toner with charges sufficiently, thereby, the addition of additives into toner for the improvement of chargeability is not almost needed. Therefore, it become possible to achieve the essential improvement for the problems as above mentioned.
However, charging means such as a carrier, a sleeve, a doctor blade or the like must possess not only a strong ability to giving charges but also excellent permanence such as friction resistance to a toner.
Recently, the miniaturization of a developer is required for the miniaturization of a copying machine, the low cost or a multi-color printing of a copying machine. Further, a developer is required to be used in an unit system from the view point of a maintenance service. A single component developing system has been paid attention to because of those requirements as above mentioned. However, it is difficult to charge a toner uniformly, because a toner is charged at the moment the toner passes through between a toner-levelling blade and a sleeve. It is proposed to overcome these defects as above mentioned that a surface of a toner-levelling blade is coated with a resin or the like having strong chargeability. It is also proposed for further improvement of chargeability that a charge controlling agent is contained in a resin-coating layer. However, electrification build-up properties and stabilization of charges are not improved satisfactorily.